The present invention generally relates to ink jet and other types of printers and, more particularly, to a system and method for selectively warming a printhead for optimized performance.
Many modem printing devices incorporate thermal ink jet technology. Typically, this technology utilizes a printhead (also known as a pen) having a silicon die supporting one or more vaporization chambers. During operation, resistors or other ink ejection elements on the silicon die are heated in order to vaporize and eject ink through nozzles overlying the vaporization chambers, thereby causing dots of ink to be printed on a recording medium, such as paper.
In general, the temperature of a printhead will change or fluctuate while printing. When a printer is printing xe2x80x9clightxe2x80x9d areas or in a slow mode, the printhead temperature will drop; when a printer is printing xe2x80x9cdensexe2x80x9d areas or in a fast mode, the printhead temperature will rise. As the printhead temperature changes, the amount of ink ejected from any given vaporization chamber (commonly referred to as the xe2x80x9cdrop volumexe2x80x9d) will also change. In particular, a rise in printhead temperature causes an increase in drop volume, while a fall in temperature causes a decrease in drop volume.
This relationship between temperature and drop volume can affect the print quality of a printer. Specifically, large changes in printhead temperature may cause thermal bandingxe2x80x94i.e., a shift in color saturation or hue attributable to changes in drop volume. Thermal banding is very pronounced in, for example, a sign plot, as illustrated in FIG. 1, where a dark background is printed on a lightly-colored medium to form a large, lightly-colored letter.
FIG. 1 depicts a lightly-colored letter xe2x80x9cIxe2x80x9d bordered by a darkly-colored background. In an exemplary operation for the printing of such letter using a large format printer, a recording medium is incrementally advanced from left to right (as indicated by arrow 1 designated xe2x80x9cPaper Advancexe2x80x9d). During each increment, a printhead is scanned from bottom to top (as indicated by the arrow 2 designated xe2x80x9cPrint Axisxe2x80x9d) as it is being energized to deposit a swath of ink thereon. In the scans where no portion of the letter xe2x80x9cIxe2x80x9d is present, such as at areas 3 and 4, the printhead is fired very rapidly for a relatively long period. This causes the temperature of the printhead to rise, which, in turn, increases the drop volume. Accordingly, areas 3 and 4 appear dark, with deep color saturation into the recording medium. In contrast, for scans in which a portion of the letter is formed, such as at area 5, the printhead is fired, on average, less frequently. The temperature of the printhead remains relatively cool, and thus, less ink is deposited onto the medium. Accordingly, this area appears faded or xe2x80x9cwashed outxe2x80x9d because color saturation is not as great. The noticeable difference in color saturation between areas with and without letters is considered to be a serious printing defect.
To mitigate the problem described above, a number of techniques have been previously developed. According to one previous technique, commonly known as pre-swath warming (PSW), the temperature of a printhead is raised to the same, predetermined value at the beginning of each scan of the printhead. This could be accomplished by xe2x80x9cdry firingxe2x80x9d the resistors on a silicon die of the printhead to produce heat. However, because the printhead temperature is allowed to vary during each scan (after having been heated to the same, predetermined temperature at the beginning of each scan), the PSW technique does not control temperature adequately enough to mitigate the effects of thermal banding.
According to another previous technique, commonly referred to as smart pulse warming (SPW), the printhead temperature is continuously measured and warmed to a fixed, predetermined value at all times during printing. This value is approximately equal to a temperature experienced during maximum print densities. Like the PSW technique, the SPW technique also heats the printhead by dry firing resistors on the silicon die. The fixed printhead temperature under the SPW technique is required to be set at a value well-above common operating temperatures. This is problematic in that the continuous operation of a printhead at a high temperature adversely impacts upon the life of the printhead and its performance.
The disadvantages and problems associated with controlling or regulating the temperature of a printhead to mitigate the effects of thermal banding have been substantially reduced or eliminated using the present invention.
In accordance with one embodiment of the present invention, a system includes an operating condition sensor for generating a present value corresponding to an operating condition (e.g., printing density, print speed, etc.) within a printer. A memory stores at least one historical value for the operating condition. A controller, coupled to the operating condition sensor and the memory, calculates the amount of change in the operating condition between the present and historical values. The controller then initiates warming of a printhead in response to the calculated amount of change.
Important technical advantages of the present invention include selectively warming a printhead in response to relatively sudden changes in the operating conditions and/or characteristics of a printer, wherein a printer condition is something which can be detected and a printer characteristic is something which can be calculated or predicted from one or more printer conditions. This selective warming avoids thermal banding. Specifically, a printhead is warmed only upon the occurrence, within a predefined interval, of at least a predetermined amount of change in any one, or a combination, of various printer conditions or characteristics, such as duty cycle, print density, print speed, and the like, which can be empirically detected, calculated, or predicted. The print speed of a printer can be fast or slow, depending upon the quality of print desired. The duty cycle is a measurement derived from, among other thing, the print density and the number of scans. Because sudden changes in printing conditions and characteristics generally do not occur frequently, the printhead will not be warmed at all for much of its operating time. This reduces dramatic fluctuations in drop volume, while not maintaining a printhead at an undesirably high temperature. Accordingly, the present invention mitigates the effects of thermal banding without adversely impacting upon the life and performance of a printhead. Other important technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims.